The present invention relates to exhaust gas recirculation valves for internal combustion engines; more particularly, to solenoid actuators for such valves; and most particularly, to resilient means for reducing hysteresis in the response of such actuators.
Exhaust recirculation valves for internal combustion engines are well known. Such a valve typically includes a poppet-type valve mechanism actuated linearly by an electric solenoid in response to signals from an engine control module. The solenoid includes a central armature attached to the valve poppet. The armature is actuated by being drawn axially of primary and secondary electromagnetic pole pieces. Typically, the armature slides within, and is guided by, a non-ferromagnetic tube which also functions as a virtual xe2x80x9cair gapxe2x80x9d to enhance the axial force on the armature. The tube is pressed into the pole pieces and ideally remains stationary while the armature is reciprocated within. However, because of manufacturing tolerances, the axial space allotted for the guide tube must be slightly longer than the tube itself such that typically a small gap exists at one end or the other of the tube. Thus, in prior art solenoids, the tube undesirably may be moved axially of the pole pieces due to friction with the moving armature during the duty cycle of the armature.
The tube may be provided as a cup-shaped element having a blind end against which the armature instantaneously comes to rest before reversing its motion. This hard stop for the armature can result in undesirably rapid wear of the stop surface and of the armature itself. The impact force in an automotive solenoid may exceed 100 or more times the force of gravity. Further, because the guide and armature share very close tolerances and cannot be perfectly cylindrical, and because the closed end of the tube may be deformed from cylindrical ideality by the deep-drawing process for forming the tube, the armature typically tends to stick slightly as it approaches the blind end of the tube within the primary pole piece. Also, a phenomenon known in the art as xe2x80x9cmagnetic latchingxe2x80x9d can cause the armature to stick, momentarily, against the pole piece. These occurrences can cause undesirable hysteresis in the duty cycle of the armature.
What is needed is a means for softening the impact of the armature against the inward stop of the guide tube, for preventing the guide tube from slipping axially of the pole pieces, and for reducing hysteresis in the duty cycle of the armature.
It is a principal object of the present invention to reduce hysteresis in the duty cycle of a solenoid armature, thereby improving position control of an attached poppet valve.
It is a further object of the invention to prevent the guide tube from slipping axially within the pole pieces of a solenoid.
It is a still further object of the invention to soften the impact of the armature against the inward stop of the guide tube to extend the working life of the solenoid.
Briefly described, a solenoid assembly in accordance with the invention includes a Hookian-like deformable element, for example, an undulated washer, disposed between the closed end of an armature guide tube and the primary or inward pole piece. The element is resilient and has a spring constant selected to 1) prevent axial motion of the guide tube due to frictional drag on the solenoid armature moving within the guide tube; 2) absorb and store a large percentage of the kinetic energy transferred by the armature impacting the end of the guide tube and reversing its motion; and 3) resiliently return a percentage of the stored energy to assist the armature in accelerating in the reverse direction. The element may take the form, for example, of a wavy-washer, a resilient mesh ring, or circular corrugation formed in the end of the tubular guide itself.